The present invention relates to a method for manufacturing a gravure-transfer-coated steel plate.
Coated steel plates are known which are manufactured by forming patterns over a continuous steel plate wound in the form of a coil. For conventional methods of manufacturing such coated steel plates, there are those using a simple coating method and those using an offset printing method. In the former case using a simple coating method, it is difficult to form diverse patterns over a steel plate because the coating method used is simple. On the other hand, in the latter case using an offset printing method, it has been industrially implemented only for limited patterns such as patterns of wood textures.
Meanwhile, Japanese Laid-open Patent Publication No. Sho. 58-205567 discloses a method for manufacturing a coated steel plate by forming a primer layer (a lower coating) over a continuous metal plate wound in the form of a coil, by use of double coating and double baking processes, pressing a thermal transfer sheet onto the primer layer on the metal plate, and then forming an upper coating over the thermal transfer sheet.
Also, Japanese Laid-open Patent Publication No. Hei. 4-280994 discloses a method for manufacturing a transfer-coated steel plate using a thermal pressing transfer process. The disclosed method is adapted to reduce the surface roughness of a steel plate applied, thereby obtaining a high brightness. In accordance with this method, coated steel plates free of coating defects can be manufactured even when a thin film coating is carried out using a roll coating process.
However, the above mentioned method involves drawbacks in that it is necessary to use steel plates exhibiting a high brightness, that the primer layer (the lower coating) adapted to provide a desired corrosion resistance and a desired surface smoothness is formed using double coating and double baking processes, that the thermal transfer temperature used is lower than that in a general PCM(PRE-COATED METAL) color line, and that the line speed is low, for example, 5 to 40 MPM.
Recently, laminar steel plates have been commercially available which are formed by printing diverse patterns over a film, and then laminating those patterns over a steel plate. In such plate products, there is a problem in that chlorine or dioxine gas, which are undesirable in terms of their negative effect on the environment, may be generated when those plate products are melted for the regeneration of metal plates.
The present invention has been made in view of the above mentioned problems, and an object of the invention is to provide a method for manufacturing a gravure-transfer-coated steel plate, which is capable of transferring a pattern from a gravure transfer sheet over a metal plate without any additional adjustment for processing conditions of a general PCM color line, thereby producing a gravure-transfer-coated steel plate having a beautiful appearance and a high workability.
In accordance with the present invention, this object is accomplished by providing a method for manufacturing a continuous gravure-transfer-coated metal plate by transferring a pattern over a continuous metal plate wound in the form of a coil using a gravure transfer sheet, comprising the steps of: coating, over the metal plate, a primer coating material essentially consisting of 20 to 40% of a polyester resin having a number average molecular weight of 5,000 to 20,000 while exhibiting a glass transition temperature of 10 to 50xc2x0 C., 4 to 15% of a melamine resin, 1 to 10% of a urethane resin, 0.3 to 3% of isocyanate, 15 to 35% of a pigment, 30 to 50% of a hydrocarbon and ester-based mixed solvent, and 1.5 to 3% of additives; heating and setting the primer coating layer in a hot-air-blowing dry oven, thereby forming a lower coating layer; thermally pressing a gravure transfer sheet, printed with a desired pattern on a base film layer thereof, onto the lower coating layer; releasing the base film of the gravure transfer sheet from the lower coating layer, thereby transferring the pattern, as a transfer coating layer, to the lower coating layer; coating, over the transfer coating layer, an upper coating material selected from the group consisting of a thermosetting fluorine resin, a thermosetting polyester resin, and an ultraviolet-thermosetting polyurethane-acryl-based resin; and heating and setting the upper coating layer or radiating ultraviolet rays onto the upper coating layer, thereby forming an upper coating layer.
The substrate metal plate used in the method of the present invention may include any kind of known steel plates, for example, an aluminum plated steel plate, a zinc-aluminum alloy hot dipped steel plate, a copper plated steel plate, a tin plated steel plate, a chromium plated steel plate, a nickel plated steel plate, and a zinc galvanized and hot dipped steel plate.
The substrate metal plate may be subjected to a chromate treatment in order to improve the corrosion resistance of the metal plate and the bondability of the primer coating (lower coating) material to the metal plate. Preferably, the chromate treatment is carried out by coating a chromate layer over the substrate metal plate.
The primer coating layer functions to improve the bondability of the transfer coating layer to the surface of the metal plate. In some cases, the primer coating layer also functions to provide a background color for the transfer coating layer (transfer pattern). The primer coating layer has an inseparable relation with the transfer coating layer to be formed thereover. Where there is an undesirable characteristic difference between the primer coating layer and transfer coating layer, problems may occur in terms of the bondability between those coating layers and the hardness of the coating layers. A coating crack and a yellowing phenomenon caused by ultraviolet rays may also occur. For the primer coating material, those should be used which are free of an occurrence of popping when they are dried by a flow of blown hot air in a subsequent setting process conducted after the coating process while exhibiting a superior bondability to the transfer coating layer, a superior workability, a high weather resistance, and a high corrosion resistance.
The primer coating material contains, as a major resin thereof, a flexible polyester resin having a number average molecular weight of 5,000 to 20,000 while exhibiting a glass transition temperature of 10 to 50xc2x0 C. The reason why the flexible polyester resin is used is to allow the primer coating material to exhibit a high elongation, in addition to the basic physical properties of the lower coating material such as corrosion resistance, bondability to the transfer coating layer and plated metal plate, and surface smoothness, thereby exhibiting a sufficient buffering function at the interface between the metal plate and transfer coating layer to allow cracks formed at the metal plate during a treatment for the metal plate to be absorbed by the lower coating layer without being propagated to the transfer coating layer. In order to allow the polyester resin to exhibit a maximum elongation, it is necessary to inhibit the formation of functional groups. To this end, polyhydroxyl alcohol such as ethylene glycol is used in the synthesis of the polyester resin to allow the functional groups, such as hydroxyl groups (xe2x80x94OH) or carboxyl groups (xe2x80x94COOH), branched to the main chain of the polyester resin to have a linear structure.
The primer coating material also contains a crosslinking resin (thermosetting resin) for a coating formation. A mixed resin of methylated and buthylated melamine resins may be used for the crosslinking resin. For such a melamine resin, RESIMINE 755, RESIMINE 757, RESIMINE 751 (manufactured by SOOLUTIA Company, U.S.A.), CYMEL 1168, CYMEL 1170, and CYMEL 232 (CYTEL Company, U.S.A.) are commercially available. The primer coating material also contains a setting promoter which may include a sulfonic acid masked with an epoxy resin, such as P-toluene sulfonic acid or dinonyl naphthalene sulfonic acid. The setting promoter is used in an amount of 0.3 to 3 parts by weight based on the weight of the primer coating material. Preferably, the setting promoter is used in an amount of 0.5 to 2 parts by weight. Where the setting promoter is used in an amount of less than 0.3 parts by weight, an insufficient setting of the coating may occur under certain conditions. Where the amount of the setting promoter exceeds 3 parts by weight, the coating is too rapidly set, so that a popping or shrinkage phenomenon may occur in the coating.
In order to achieve an improvement in bondability, an assistant crosslinking resin may also be used. For the assistant crosslinking resin, a non-yellowing isocianate such as a masked hexamethylene di-isocianate may be used. The assistant crosslinking resin is used in an amount of 1 to 10 parts by weight based on the weight of the primer coating material. Preferably, the assistant crosslinking resin is used in an amount of 3 to 7 parts by weight. Where the assistant crosslinking resin is used in an amount of less than 1 part by weight, the effect of improving the bondability of the coating is degraded. Where the amount of the assistant crosslinking resin exceeds 10 parts by weight, an increase in the cost of the coating material occurs.
A catalyst for the assistant crosslinking resin is also used in an amount of 0.5 to 1 part by weight, preferably 0.5 to 1 part by weight, based on the weight of the primer coating material. Where the catalyst is used in an amount of less than 0.1 part by weight, its effect is degraded, thereby degrading the effect of improving the bondability. On the other hand, where the amount of the catalyst is more than 2 parts by weight, a yellowing phenomenon occurs. In the latter case, an increase in the cost of the coating material also occurs.
Where the substrate, to which the primer coating material is applied, is of the interior, the primer coating material is added with a TiO2 white pigment. On the other hand, the substrate is of the exterior, an anti-corrosive pigment is added to the primer coating material in order to achieve an improvement in corrosion resistance. Of course, the anti-corrosive pigment may be used irrespective of the interior or exterior substrate, if desired by the consumer. If necessary, an ultraviolet filler may also be added for an enhancement in anti-ultraviolet property.
A preferable composition of the primer coating material is described in the following Table 1.
The gravure transfer sheet includes a base film printed with a desired pattern and a desired color. The pattern of the gravure transfer sheet is transferred to a steel plate coated with a primer using heat and pressure. Thus, color steel plates printed with diverse patterns can be produced.
The base film of the transfer sheet used in accordance with the present invention may have a single-layer structure consisting of a plastic film exhibiting a relatively high heat resistance, such as a polypropylene film or a PET film. Alternatively, the base film may have a multi-layer structure consisting of an optional substrate, and the above mentioned plastic film laminated over the optional substrate. The pattern and color of the transfer sheet is printed using a transfer ink. Transfer sheets for metal, which are cooperatively manufactured by YUNHAP Steel Industry Co., Ltd., in Pusan, Korea, and Sunjin Co., Ltd., in Seoul, Korea, and use a PET film as a base film, may be commercially available for the transfer sheet. The transfer sheet has a multi-layer structure mainly including five layers in terms of the function thereof. That is, the transfer sheet has a base film (PET film), and a protective layer for allowing an easy release of the base film while protecting a printed layer. The printed layer, which is also included in the transfer sheet, is printed with a desired pattern and a desired color. The transfer sheet also has a reinforcing layer for hiding and reinforcing the metal plate, onto which the printed layer is to be transferred, and an adhesive layer for enhancing the bonding force of the printed layer to the primer layer.
The upper coating layer coated over the transfer coating layer may be made of a thermosetting fluorine resin, a thermosetting polyester resin, or an ultraviolet-thermosetting polyurethane-acryl-based resin. Preferably, the upper coating layer is made of polyester resin having a number average molecular weight of 5,000 to 20,000 while exhibiting a glass transition temperature of 40 to 70xc2x0 C., in terms of the costs. Taking into consideration the reactivity of the upper coating material to the transfer coating layer and the ultraviolet resistance of the upper coating material, it is necessary to limitatively select the solvent, antifoaming agent, and crosslinking agent used. Where the metal plate is used for exterior purposes, it is desirable to use an ultraviolet filler. Where a polyester resin is used for the upper coating material, it is preferable to use a mixture of a hydrocarbon-based solvent and an ester-based solvent. For the crosslinking resin, it is preferable to use melamine and urethane.